This invention relates to portable radio telephones and particularly but not exclusively to such telephones for communication with base stations in a digital cellular radio telephone system employing transmission by a plurality of carrier frequencies in frames, such as TDMA frames, each consisting of a predetermined number of time slots. The invention also relates to a method of operation of such radio telephones, frequently called handsets.
Each cell of a cellular system is supported by a base station. Each base station has a maximum capacity. In order to increase the capacity of the system another base station can be added. This increases the maximum number of concurrent users. In systems in which two base stations can operate with overlapping cells, more than one base station can provide coverage for a particular area if the concentration of users is high.
Handsets are in communication with a base station via an available over the air communication channel. Allocation of a channel to a handset when a connection is first required is called "call set-up". In order to set up a call the handset must be in `idle lock` i.e. monitoring the system with no call in progress. When a traffic channel has been assigned the handset is in `active lock`. When a change in channel is required during a call to maintain call quality this is termed "handover". Handover can either be to a different channel at the same base station (intracell) or to a different channel at a different base station (intercell).
If a handset carries channel selection algorithms, in both idle and active lock the handset scans the available channels to gain information useful for call set up or handover. The Radio Signal Strength Indicator (RSSI) of each channel is monitored at regular time intervals and the measured signal strengths are stored in a channel list.
The RSSI information indicates to the handset which channels have bearers. A high RSSI indicates a bearer. This is not foolproof as a high RSSI may also be the result of noise or interference between channels but it is a good guide. The RSSI information alone does not indicate which base station is transmitting on a particular channel.
A handset will scan the bearer channels periodically to determine the RSSI information necessary for call set up or handover. Once it is determined that a new channel is needed either for call set up or for handover, the handset uses the RSSI information from the most recent scan.
In order to select a traffic channel the handset must identify both the channel and base station. A problem may arise when a handset needs to open a communication channel through a base station other than the one to which it is currently locked i.e. the current base station. This may be necessary, for example, if the current base station is at capacity. The problem occurs because a handset will typically only look for the strongest base station as an alternative to the current base station for a new channel.
Using the RSSI information gathered periodically the strongest base station can be identified by monitoring the channel with the highest RSSI. A quiet channel can be selected on that base station by selecting a channel with a low RSSI. In the absence of additional information it is not possible to select a base station other than the current or strongest base station. Once a handset has determined that the base station to which it is locked is at capacity, it is a matter of chance whether or not there is a stronger base station with which the handset can attempt call set up or handover. If the current base station is the strongest, no other base station can be selected. If another base station is stronger and that is also at capacity no other base stations can be selected with the information available to the handset.
The handset's options for channel selection are, therefore, limited to channels on the current, or strongest base station if this is different to the current base station. This has severe consequences when either the current base station is strongest and at capacity or both the current and the strongest base stations are at capacity, as this results in the handset in idle lock being unable to make or receive a call. It also means a handset in active lock will lose a call-in-progress if the channel deteriorates unless this is the result of the handset moving out of the range of one base station into the range of another.